Adjusting Dough Rheology

Bakers have long tried to reduce the level of art in baking created by the interactions of flour, water and other ingredients under different processing conditions. However, rheology is a complex physical property, and dough rheology is no exception. Scientists have designed instruments to record mixing, water absorption, enzymatic activity, fermentation and oxidation properties of flours. Suppliers have designed ingredients to enhance the mixing and machine-ability of dough and give it more elasticity and strength. It’s the combination of all these variables that allow the baker to have some control over dough rheology. Using the science and a little art will help optimize dough for ease of processing and ensure the right finished- product attributes.

Taking dough’s measure

Rheological testing plays an important role in maintaining consistent flour quality. There are two primary categories of physical testing of doughs: the torque- or viscosity-measuring instruments like the farinograph and mixograph, and the elasticity-measuring instruments like the alveograph.

A farinograph is one of the most commonly used instruments. This high-speed mixer measures the resistance of the dough against constant mechanical shear. It measures the rate of flour hydration, dough development time (peak time), flour mixing tolerance and flour strength. The mixograph, a miniature high-speed-recording dough mixer, uses four vertical pins attached to a rotating mixer head. As gluten develops and dough consistency increases, it requires more force to push the pins through the dough. The force creates a twisting motion on the bowl and the instrument measures the torque. The mixograph provides the mixing time to peak dough development, and the mixing tolerance. The alveograph measures resistance to expansion and the extensibility of a dough. One shortcoming of this method is that it uses a constant water absorption for all doughs. The instrument is not considered the best way to assess hard wheat flours.

These measurements, along with baking tests, usually provide information about the flour’s baking behavior. A combination of tests helps the baker optimize the mixing, oxidation, fermentation and enzymatic treatments for each flour. Mixing is an important step in the bread process because of its role in dough development. Mixing time for optimum dough development is difficult to assess under typical plant conditions, because a number of factors influence it: flour type, dough temperature, mixing speed, level of water absorption, oxidative treatment, enzyme supplementation and more. Flours can differ considerably in their mixing requirements for achieving optimum machineability and finished product attributes.

Mixing up research

Rebekah McIntier, a food science graduate student at the University of Wisconsin-Madison, has focused her studies on determining the relationship between the intensity of mixing, dough development and rheology. Her hypothesis is: “The mixing speed where energy input to reach peak torque becomes independent of the mixing speed is a point where the mixers have the same intensity.”

McIntier’s initial research was done with flour and water doughs made with hard, spring or soft flours (straight grade and untreated). She used measurements from a farinograph (C.W. Brabender, Hackensack, NJ) and a reomixer (Reologica Instruments AB, Lund, Sweden), a pin mixer similar to a mixograph. The average number of revolutions to peak consistency was measured for each flour, using both mixers. All samples were mixed greater than or equal to a specified number of revolutions to reach a constant strain or peak consistency. The research concluded that there was a dependence of dough development on mixing speed at low speeds. Dough development was independent of mixing at high speeds. The minimum critical speed for the farinograph was determined for the hard- and medium-strength flours. The reomixer did not yield a true measure of actual energy, so it was difficult to make mixer comparisons, but the recent addition of a torque sensor should to yield more comparable results in the near future.
The remaining part of McIntier’s research will focus on yeast doughs, measuring the dough development by time to peak (highest point on a torque curve) while mixing at different speeds, and the amount of energy input. A farinograph and pin mixer will be used for the torque measurements. A Kieffer rig used on a texture analyzer measures the elongational viscosity, or dough strength. A portion of the dough is then used for bake tests to measure the loaf volume and crumb texture.

Another method summarized in a patent filed in 2003 by Richard Dempster, Maureen Olewnik and Virgal Smail of the American Institute of Baking, Manhattan, KS, is the “determination of dough development using near-infrared radiation.” The method provides a quick and accurate means of estimating dough development times in a commercial baking facility with many different lots of wheat flour. It works by directing near-infrared radiation against a dough formulation during mixing. The time-dependent absorbance spectra are collected and then analyzed by calculating magnitude ratios at predetermined spectral absorbances, and then estimating the dough development time as a function of the magnitude ratios.

Emulsifying effects

Ingredients, too, can influence dough’s physical characteristics. Specific emulsifiers, often referred to as dough strengtheners or conditioners, provide benefits to yeast-raised dough through their interactions with the gluten proteins. They provide improved mixing tolerance, increased water absorption, better gas retention during fermentation, proofing and conveying, plus shorter proof times. Traditional dough strengtheners include stearoyl lactylates, diacetyl tartaric acid esters of mono- and diglycerides (DATEM), succinylated monoglycerides (SMG) and ethoxylated monoglycerides (EMG). Typical use levels for these emulsifiers ranges from 0.25% to 0.50%. Combining sodium stearoyl-2-lactylate and calcium stearoyl-2-lactylate complex with gluten during mixing provides improved side-wall strength and protection against dough collapse during bread manufacture. Both of these emulsifiers provide excellent dough strengthening and good crumb softening. DATEM will also provide excellent dough-strengthening properties and improved loaf volume but fair crumb softening.

“DATEM facilitates the aggregation of gluten proteins, creating a gluten network that will entrain air better and result in better bread volume and crumb texture,” says David Kappelman, group manager/bakery, Danisco USA, Inc., New Century, KS. “It will add improved tolerance to the abuses normally seen in high-speed bread production. It will also have a drying effect on the dough, allowing for improved machineability.” SMG is a good dough strengthener, providing good oven spring and some crumb softening. EMG provides good dough strength, but little crumb softening.

Strengthening plus

Protein ingredients can be included in the dough conditioner category because they add strength and durability to doughs. Generally, the higher the level of protein in the dough, the greater the strength. For bread doughs, vital wheat gluten has long been used for this purpose. Vital wheat gluten, which is the insoluble protein fraction extracted from wheat flour, contains 75% to 80% protein. Adding it to formulas increases the protein content and absorption of flour and increases the dough tolerance. Wheat protein isolate, containing 90% protein, is a more recently available wheat protein ingredient that positively affects dough rheology. “Our Arise® line of specialty wheat protein isolates adds elasticity, extensibility and uniformity to dough, while also extending its life and machineability,” says Steve Ham, director of marketing specialty ingredients, MGP Ingredients, Atchison, KS. It also provides benefits for the mixing stage.
“The wheat protein isolate mixes in with the longer gluten chains of the dough and shortens the mix time,” adds Topher Dohl, application technologist, MGP Ingredients. This wheat protein isolate will provide benefits to a variety of applications. “Arise provides optimum extensibility and strength to tortilla, cracker, pizza, bagel and bread dough.”

Extensibility can be improved with low usage rates of the isolate. A 0.5% to 3.0% addition of this ingredient per cwt of flour in a pizza dough improves elasticity of the dough and reduces shrinkage, says Ham. The improvements in dough extensibility are measurable.

“In some comparison experiments with vital wheat gluten, Arise provides more extensible doughs as measured by a mixograph,” says Dohl. “Extensibility was also measured by the Kieffer method on a texture analyzer.”

“It can also be used as a partial egg-white replacement in dry, fresh and retorted pasta and helps to reduce stickiness,” says Ham. Hydrolyzing the protein further enhances the functionality of the wheat protein isolate. “Our hydrolyzed wheat protein isolate develops dough quicker and provides a clean label compared to L-cysteine or bromate for products like bagels, pizza crusts or tortillas,” he says.

Sometimes it’s useful to have the flexibility of adding a new ingredient without affecting dough rheology. Many formulators would like to add fiber to doughs without changing the consistency and machineability of the dough. Most fiber ingredients are very good at absorbing water, which can change the physical properties of the dough significantly. “Fibersym™ has 70% minimum dietary fiber and holds 0.7% times the weight of flour, so it doesn’t change the dough properties like other fibers, which absorb much more water,” says Dohl.

This resistant wheat starch has a wide range of uses, along with a neutral taste and white color, “making it a versatile and easy to incorporate ingredient for breads, buns, muffins, cakes, pizza doughs and extruded snacks,” says Ham.

Reducing more problems

Reducing agents also impact rheology and fall into the dough conditioner category. These ingredients— L-cysteine, sorbic acid, and the sodium salts of sulfite, bisulfite and metabisulfite—can reduce mixing time and help develop more pliable, extensible doughs. Extensibility, strength and tolerance of bread doughs depend on flour quality, water absorption and mixing conditions. Mixing dough stretches the gluten in the flour and pulls it apart. The gluten is reformed during proofing and baking, which yields the final structure and strength. Reducing agents reversibly break down the gluten and, once they are used up, the gluten reforms, similar to the action of a mixer. They are typically used with a combination of high-strength flour and high-speed processes to improve machineablity and loaf volume, but they also help reduce mix time and energy input. Frozen bread doughs benefit from reducing agents because a shorter mix time helps to improve yeast stability. Reducing agents can also decrease the elasticity that can cause dough shrinkage or curling of pizza, tortilla, cookie and cracker doughs.

Glutathione, yeast and L-cysteine are protein-based reducing agents. Cysteine, or L-cysteine hydrochloride, a synthetically produced amino acid, is the most commonly used reducing agent in bread. It is added at the mixing stage and works quickly. Glutathione, a cysteine-containing peptide that is not commercially available functions like L-cysteine, but is more effective because it can react many more times. A natural source of glutathione is yeast. Nonleavening yeasts can be used as reducing agents in the same applications that use Lcysteine. The gluten proteins of flour also contain cysteine. The sulfhydryl group in cysteine can react (oxidize) with another cysteine molecule to form a disulfide bond to make one molecule of cystine. When gluten molecules react with one another (become oxidized) during bread manufacture, dough strength increases, but extensibility decreases. The mixing process mechanically breaks these bonds to improve the extensibility needed for moulding, and the gluten later reforms during proofing and baking. The disulfide bonds formed in gluten can also be broken through disulfide interchange with cysteine or glutathione. The number of crosslinks in the gluten will be broken proportional to the number of cysteine or glutathione molecules added.

Sulfites are commonly used in cookies and crackers. The bisulfite ion derived from sulfur dioxide or sodium bisulfite is the active ingredient. Sulfites have some negative reactions associated with their use, including destroying thiamine, inhibiting yeast and causing sensitivity reactions in people. If used at levels greater than 10 parts per million in the United States, the FDA requires a special label declaration.

Ascorbic acid is used as a reducing agent in applications with closed continuous mixing. It functions as a reducing agent without the presence of oxygen, but will act as an oxidizing agent with oxygen present. A coated form can delay the reaction in a process.

Oxidizing options

Dough conditioners like bromates, iodates, ascorbic acid, azodicarbonamide (ADA) and calcium peroxide are generally known as oxidants. They act on the gluten in flour to increase its strength and produce moreuniform and higher-volume finished products. The oxidant’s rate of reaction during dough processing is important to consider. A faster-reacting oxidant like ADA could provide more dough strength at the mixing stage, while a slower-reacting oxidant like potassium bromate could provide strength during processing. Potassium bromate works well as the sole oxidant because of its slow action, high tolerance and good oven spring. Bromates and iodates are not allowed in Europe and are less popular in the United States because they are considered potential carcinogens. Generally, the need for an oxidant is directly related to the amount of mechanical processing of the dough; therefore, a continuous mixing process would require a higher level of oxidation than a straight dough process. Combinations of these oxidants can provide adequate dough strength throughout the entire process.

Other ingredients can affect the requirements of an oxidizing agent. Reducing agents can interact with the oxidants and create a need for greater levels of oxidants. Other ingredients that affect oxidation include dead yeast cells, wheat germ, non-wheat flours, non-heat treated milk and different flour types.

Scientists have proposed many mechanisms of how oxidants function. Among the discarded theories are inhibition of proteases, oxidation of thiols, affecting protein aggregation and the ascorbic-acid mechanism. The favored mechanism is thiol-disulfide interchange. Insufficient oxidation makes a dough weak, extensible, soft and sticky, and gives it poor machineability. Use rates for oxidants vary between 10 and 75 parts per million.

Acting as a catalyst

Enzymes are proteins that react as natural catalysts to create or enhance dough reactions. The main reason for using enzymes in dough processing is to control dough properties. Enzymes are very specific to the reactions they catalyze, but the variety of enzymes used as dough conditioners provides many functions. Proteases are one of the most commonly used enzymes to control dough characteristics in yeast-raised products. Proteases split peptide linkages in gluten to relax the dough. Proteases are added in the mixing stage, and activity continues throughout fermentation until the heat of the oven inactivates the enzyme. The effect of the enzyme depends on reaction time allowed, as well as the pH. Different proteases will have different optimum pH conditions. Manufacturers can reduce mixing time to reach an optimum dough by as much as 25% with the addition of a protease at the right temperature, pH and time. A protease can also relax a dough, so it improves sheeting and machineability. The improved extensibility will also enhance gas retention and provide better pan flow, so buns and rolls will flow during proofing and baking.

Uninhibited xylanase is another functional enzyme for dough processing. “Xylanase will solubilize arabinoxylans, allowing for improved gluten development, which results in higher quality finished bread,” says Kappelman. Xylanases also create drier, more extensible doughs, which sheet more easily through the moulding process. “Our uninhibited xylanase is unique in that it is not affected by the endogenous xylanase inhibitors found in wheat flour,” he continues. “This characteristic creates dramatically improved performance and consistency in functionality compared to other xylanases.”

The processing and ingredient knowledge is available today to achieve the optimum dough rheology required for any baked product. The information is waiting for application to your next new product.